1. Field of the Invention
This invention relates to security systems and to power door lock systems for motor vehicles, and more specifically to a device for preventing an operator of the vehicle from locking an ignition key in the vehicle when the key is left in the ignition.
2. Description of the Related Art
Systems which provide an audible warning when a motor vehicle door is open and an ignition key is left in the ignition are well-known. However, because such conventional key-in-ignition warning systems do not prevent an inattentive operator from locking the ignition key in his or her vehicle despite the warning, attempts have been made to provide a system which not only warns the operator, but also prevents the door from locking whenever an attempt is made to leave the vehicle with the key in the ignition.
An example of a system which prevents the operator from locking the vehicle when a key is in the ignition is described in U.S. Pat. No. 4,709,777 to Metz. A key-in-ignition sensor switch and a door-open sensor switch are each connected to a logic circuit which outputs an electrical signal when the key-in-ignition sensor switch and door-open sensor switch indicate simultaneous key-in-ignition and door-open conditions. An open door generally indicates that the operator is attempting to leave the motor vehicle.
The signal output by the logic circuit opens a relay which in turn opens a lock circuit connected to a power door lock/unlock mechanism. The lock circuit normally actuates a solenoid or motor for locking the door when a lock switch is manually pressed. However, when the logic circuit signal is output in response to simultaneous door-open and key-in-ignition conditions, the open relay prevents actuation of the locking motor or solenoid and instead closes an alarm circuit for activating an audible alarm, for example, the vehicle's horn or a buzzer. Thus, locking of the door is prevented whenever the key-in-ignition and door-open conditions are met.
A similar type of door locking prevention system is disclosed in U.S. Pat. No. 4,223,296 to Kim et al. Again, a door-open sensor switch and a key-in-ignition sensor switch are electrically connected to output a signal indicative of a simultaneous key-in-ignition and door-open condition, but instead of a relay for opening the door lock circuit, the system described by Kim et al. requires a soleniod which physically prevents the power door lock switch from moving to close the locking circuit, thereby preventing the door from locking.
Finally, U.S. Pat. No. 4,572,320 to Robbins discloses a third type of system which, unlike the other two types of systems, allows the door to be locked but subsequently unlocks it. A key-in-ignition and a door-open switch are again provided, but both switches are connected in series with a third switch for sensing locking of the door. The third switch is a contact-type switch closed by movement of the manual door lock switch. When all three switches are closed, the third switch indicating that the door has been locked, a solenoid is activated to physically move the manual power door lock switch to a position at which it closes an unlocking circuit for unlocking the door.
While each of the above-described systems appears to be relatively effective in preventing operator lock-outs, their application is limited because of the high cost of providing the additional relays, solenoids, and other mechanical parts required to implement the systems, and because each system requires substantial modification of existing power door lock circuits in order to control and provide power to the additional electro-mechanical parts.